Question

what would happen if the aluminium leaves in an electroscope are replaced by plastic leaves

Answer 1

When you bring the negatively charged object near the top terminal, electrons will flow away from the top terminal, causing the leaves to become less positively charged and so the leaves will drop closer to each other.

If the charge on the negative object is very large compared to the positive charge on the electroscope, as you bring the negative object closer you should see the leaves initially drop to each other, then rise again as the leaves become negatively charged due to so many electrons being repelled from the top terminal of the gold leaf electroscope.